Gas chromatography is an analytical technique for both qualitative and quantitative analysis of solid, liquid and gaseous mixtures. This technique is important in both laboratory and industrial settings since it permits the components present in a sample to be analyzed. There are numerous ways in which samples for chromatographic analysis may be obtained. One technique, known as headspace sampling, provides gaseous phase samples from either liquids or solids that are sealed in a sample container. Typically, the sample container that holds the samples is hermetically sealed inside a chamber and heated or cooled to a predetermined temperature. Under suitable conditions, material contained or dissolved in the sample will form a gaseous phase sample. As used herein, the term "headspace" refers to that portion of the sealed sample container that is not occupied by the solid or liquid sample, however, the headspace may be occupied by other gasses that are displaced or mixed with the sample gasses after equilibration of the sample begins. Thus, for one example, if carbonated water in a sealed container is heated, carbon dioxide will leave the water and occupy the headspace above the liquid remaining in the partially filled container. However, in certain instances where the sample container is completely filled, the "headspace" will exist in a closed region outside the sample container.
Components of the headspace sample gasses are in dynamic equilibrium with solid and/or liquid phase of the sample remaining in the container. That is to say that molecules of each gaseous component are continuously redissolved at the same rate as identical molecules are volatilized from the sample. This equilibrium is maintained only if the concentration of headspace components, as well as other conditions such as temperature and pressure, remain constant. In standard techniques, a portion of the equilibrated headspace is removed, thereby reducing the concentration of volatilized components and disturbing the equilibrium. The effect is that while the headspace region is being sampled, additional volatile components may enter the region. Since not all components of the sample volatilize at the same rate, the collected sample will be enriched with certain more volatile components and the analysis will yield incorrect results. This is undesirable, since the goal of headspace sampling is to remove an aliquot of sample without disturbing the relative concentrations of the various components.
In prior art headspace sampling systems the sample vessels are typically sealed by a diaphragm or septum, although other types of seals are also known. A probe pierces the septum to provide a flow of sample gas from the sample headspace to the entrance of a separating column of gas chromatograph, the flow of sample gas being controlled by a valve. The entrance to the separating column is also connected via a valve to a source of carrier gas. For example, in the system disclosed in U.S. Pat. No. 4,464,940--Pospisil the carrier gas is first pumped into the headspace to create an increased pressure, a valve disconnects the carrier gas and connects the probe with the separating column, thereby allowing carrier gas plus sample vapor to flow into the separating column, i.e., from a higher pressure to a lower pressure. However, in this and other headspace techniques currently in use, the headspace is significantly disturbed--by the increased pressure caused by direct introduction of a carrier gas or otherwise--and therefore, as explained above, the sample injected into the separating column is not exactly the same as that which existed in the undisturbed headspace. The disturbance or distortion of the headspace occurs either during pressurization of the sample container or upon withdrawal through the probe piercing the septum.
Conventional headspace techniques therefore rely upon the headspace sample equilibrating at a relatively high pressure then being sampled by expanding into a sample loop or directly into the head of a column. In either case, the actual headspace gasses are diluted by expansion to a larger volume. It would therefore be desirable to provide a headspace sample at lower or equal pressure to the inlet of an analytical instrument. In such a system no expansion would occur in the process of isolating the gaseous headspace sample from the original sample.
It would also be desirable to sample gaseous phase samples from the headspace of a sealed sample container without distorting or disturbing the composition of the gasses from the sample. Thus, it is an object of the present invention to provide methods and apparatus for transferring a gaseous sample from a sealed sample container to an analytical instrument without injecting a carrier gas into the sample container.